1. Field of the Invention
The present invention relates to a process for producing an exhaust-gases-purifying catalyst. More particularly, it relates to a process for producing an exhaust-gases-purifying catalyst which can efficiently purify nitrogen oxides (NO.sub.x) in exhaust gases whose oxygen concentrations are at the stoichiometric point or more than required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) therein.
2. Description of Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and reduce NO.sub.x to purify the exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat resistant support formed of cordierite, a catalyst carrier layer formed of gamma-alumina and disposed on the support, and a noble metal catalyst ingredient selected from the group consisting of Pt, Pd and Rh and loaded on the catalyst carrier layer.
The purifying performance of the 3-way catalysts for purifying exhaust gases depends greatly on the air-fuel ratio A/F of automotive engine. For instance, when the air-fuel weight ratio is larger than 14.6, i.e., when the fuel concentration is low (or on the fuel-lean side), the oxygen concentration is high in exhaust gases. Accordingly: the oxidation reactions purifying CO and HC are active, but the reduction reactions purifying NO.sub.x are inactive. On the other hand, when the air-fuel ratio is smaller than 14.6, i.e., when the fuel concentration is higher (or on the fuel-rich side), the oxygen concentration is low in exhaust gases. Accordingly, the oxidation reactions are inactive, but the reduction reactions are active.
Moreover, when driving automobiles, especially when driving automobiles in urban areas, the automobiles are accelerated and decelerated frequently. Consequently, the air-fuel ratio varies frequently in the range of from the values adjacent to the stoichiometric point (or the theoretical air-fuel ratio: 14.6) to the fuel-rich side. In order to satisfy the low fuel consumption requirement during the driving conditions such as in the above-described urban areas, it is necessary to operate the automobiles on the fuel-lean side where the air-fuel mixture containing oxygen as excessive as possible is supplied to the engines. Hence, it has been desired to develop a catalyst which is capable of adequately purifying NO.sub.x even on the fuel-lean side (i.e., in the oxygen-rich atmospheres).
In view of the aforementioned circumstances, the applicant of the present invention and others applied for a Japanese Patent for a novel catalyst under Japanese Patent Application No. 4-130,904 (Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 which was not laid-open at the time of a Japanese Patent application corresponding to the present application). On this catalyst, there are loaded an alkaline-earth metal oxide and Pt. On the catalyst, for example, barium is loaded in the form of its simple oxide. In the catalyst, during the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving, the simple oxide reacts with NO.sub.x to form alkaline-earth metal nitrate (e.g., barium nitrate (Ba(NO.sub.3).sub.2), and the alkaline-earth metal nitrate reacts with the reducing gas such as HC and the like to be purified. As a result, the catalyst exhibits superb NO.sub.x purifying performance during the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving.
Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 also sets forth a process for producing the catalyst. That is, an active alumina coating layer is formed on a support made from cordierite or the like, and Pt is loaded on the coating layer by an ordinary method. Thereafter, the coating layer is impregnated with an aqueous solution of barium acetate or the like, dried and calcinated to load Ba or the like thereon.
However, in the case where Pt is loaded, and thereafter Ba or the like is loaded, Pt is covered with Ba or the like. Accordingly, Pt might not be able to fully exhibit its inherent functions. Namely, the catalyst might not be able to sufficiently exhibit its NO oxidation action and its NO.sub.2 reduction action. As a result, there arises a problem in that the catalyst might exhibit deteriorated NO.sub.x conversion which is lower than expected NO.sub.x conversion.
Hence, one can think of first loading Ba or the like and then loading Pt. However, this process might result in a problem in that Ba or the like, loaded on purpose, elutes into an aqueous solution for loading Pt. When loaded Ba or the like elutes into an aqueous solution of Pt compound, not only the loading amount of Ba or the like might be decreased, but also Pt might be eventually precipitated and deposited by variation of the pH value in the aqueous solution. As a result, the loading amount of Pt might be decreased.